The present invention relates to an electroacoustic transducer for converting an electrical signal into an acoustic signal, and more particularly to a loudspeaker which receives an output signal of an audio-frequency amplifier and converts the audio-frequency output signal into an acoustic signal for reproduction.
It is desirable that a loudspeaker which converts an audio-frequency signal recorded on a disc record or a magnetic tape into an acoustic signal for reproduction produces a uniform sound pressure output over a wide frequency range. In a well-known type of loudspeaker, a voice coil is arranged in an air-gap of a magnetic circuit such that it is vibrated therein, and a diaphragm is connected to the voice coil. An output signal of an audio-frequency amplifier is supplied to the voice coil to vibrate the voice coil in the air-gap to thereby vibrate the diaphragm for producing an acoustic output. The diaphragm is made of paper, metal, synthetic resin or the like and formed in cone shape or dome shape, and the voice coil is connected to the diaphragm. An outer peripheral edge of the diaphragm is supported by a soft material to form a vibration system. The vibration system has a lowest resonance frequency which is determined by a mass of the diaphragm, a mass of the voice coil, a mass of inertia of air and a stiffness of a corrugation edge or a spider which supports the diaphragm. A high-band resonance frequency of the vibration system is determined by the shape, diameter and material of the diaphragm. These resonances cause peaks in low-band and high-band frequencies on a frequency characteristic curve of the loudspeaker. A frequency range within which the loudspeaker can reproduce the acoustic signal is determined by the low-band resonance frequency and the high-band resonance frequency. The farther the distance between those frequencies are, the broader is the frequency range over which the acoustic signal of uniform sound pressure level is reproduced. Accordingly, it is preferable to raise the high-band resonance frequency in order to broaden the reproduction frequency range of the loudspeaker.
The high-band resonance of the loudspeaker is caused mainly by the resonance of the diaphragm itself. Accordingly, by raising the resonance frequency of the diaphragm itself, the high-band resonance frequency of the loudspeaker can be raised. In order to raise the resonance frequency of the diaphragm, it is preferable to form the diaphragm with a material having a high propagation speed for sound, that is, a high Young's modulus material. In this respect, the diaphragm has heretofore been formed with a high Young's modulus metal such as aluminum, titanium or beryllium or mixture of vegetable fiber and carbon fiber. However, since those materials have smaller internal loss than a paper made from vegetable fiber, the diaphragm made of those materials have higher Q (quality factor) of resonance at the high-band resonance than the diaphragm made of the paper, and produce higher peaks in the high-band frequencies of the frequency characteristic curve of the loudspeaker. As a result, the high-band frequencies of the acoustic signal reproduced by the loudspeaker is emphasized and hence the electrical signal cannot be converted into the acoustic signal with high fidelity.